Organic light emitting devices (OLEDs), which utilize thin film materials that emit light when excited by electric current, are expected to become an increasingly popular form of flat panel display technology. This is because OLEDs have a wide variety of potential applications, including cell phones, personal digital assistants (PDAs), computer displays, informational displays in vehicles, television monitors, as well as light sources for general illumination. Due to their bright colors, wide viewing angle, compatibility with full motion video, broad temperature ranges, thin and conformable form factor, low power requirements and the potential for low cost manufacturing processes, OLEDs are seen as a future replacement technology for cathode ray tubes (CRTs) and liquid crystal displays (LCDs), which currently dominate the growing $40 billion annual electronic display market. Due to their high luminous efficiencies, electrophosphorescent OLEDs are seen as having the potential to replace incandescent, and perhaps even fluorescent, lamps for certain types of applications.
Light emission from OLEDs is typically via fluorescence or phosphorescence. As used herein, the term “phosphorescence” refers to emission from a triplet excited state of an organic molecule and the term “fluorescence” refers to emission from a singlet excited state of an organic molecule.
Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are formed either as a singlet or triplet excited state, may participate in luminescence. This is because the lowest singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state. This means that, for typical phosphorescent organometallic compounds, the lowest singlet excited state may rapidly decay to the lowest triplet excited state from which the phosphorescence is produced. In contrast, only a small percentage (about 25%) of excitons in fluorescent devices are capable of producing the fluorescent luminescence that is obtained from a singlet excited state. The remaining excitons in a fluorescent device, which are produced in the lowest triplet excited state of an organic molecule, are typically not capable of being converted into the energetically unfavorable higher singlet excited states from which the fluorescence is produced. This energy, thus, becomes lost to decay processes that heat-up the device rather than emit visible light. As a consequence, since the discovery that phosphorescent materials can be used as the emissive material in highly efficient OLEDs, there is now much interest in finding still more efficient electrophosphorescent materials and OLED structures containing such materials.
White organic light-emitting devices (WOLEDs) are of interest because they offer low-cost alternatives for backlights in flat-panel displays, and may eventually find use in room or area lighting. There have been several methods for obtaining white light from organic materials. [R. S. Deshpande, V. Bulovic and S. R. Forrest, Appl. Phys. Lett. 75, 888 (1999); F. Hide, P. Kozodoy, S. P. DenBaars and A. J. Heeger, Appl. Phys. Lett. 70, 2664 (1997); and J. Kido, H. Shionoya and K. Nagai, Appl. Phys. Lett. 67, 2281 (1995)]. All of these rely on the use of a combination of several emitting materials because individual organic molecules do not span typically the entire visible spectrum from 380 nm to 780 nm. As defined by the Commission Internationale d'Eclairage (CIE), an ideal white light source has coordinates of (0.33, 0.33). Additionally, the Color Rendering Index (CRI) of a white light source is a measure of the color shift that an object undergoes when illuminated by the light source as compared with the color of the same object when illuminated by a reference source of comparable color temperature. The values of CRI range from 0 to 100, with 100 representing no shift in color. White light sources are referenced to daylight, with fluorescent bulbs typically having ratings between 60 and 90, mercury lamps near 50, and high pressure sodium lamps can have a CRI of 20. Typical luminous power efficiencies for white light sources are 15 lm/W for an incandescent light bulb and about 80 lm/W for a fluorescent lamp, not including system losses.
Over the last decade, the power (ηp) and external quantum (ηext) efficiencies of white OLEDs have been steadily improving. Electrophosphorescent OLEDs have been shown to have very high ηext when used in single emissive layer devices. [C. Adachi, M. A. Baldo, M. E. Thompson, R. C. Kwong, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett. 78, 1622 (2001); C. Adachi, M. A. Baldo, S. R. Forrest and M. E. Thompson, Appl. Phys. Lett. 77, 904 (2000); M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett. 75, 4 (1999); and M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, 395, 151 (1998)].
High efficiency organic light emitting devices (OLEDs) using the phosphorescent dopant, fac tris(2-phenylpyridine)iridium (Ir(ppy)3), have been demonstrated using several different conducting host materials. M. A. Baldo et al., Nature, vol. 395, 151 (1998); D. F. O'Brien et al., Appl. Phys. Lett., vol. 74, 442 (1999); M. A. Baldo et al., Appl. Phys. Lett., vol. 75, 4 (1999); T. Tsutsui et al., Japanese. J. Appl. Phys., Part 2, vol. 38, L1502 (1999); C. Adachi et al., Appl. Phys. Lett., vol. 77, 904 (2000); M. J. Yang et al., Japanese J. Appl. Phys., Part 2, vol. 39, L828 (2000); and C. L. Lee et al., Appl. Phys. Lett., vol. 77, 2280 (2000). Since the triplet level of the metal-ligand charge transfer state of the green-emitting Ir(ppy)3 is between 2.5 eV and 3.0 eV, deep blue fluorophores with a peak wavelength at about 400 nm, such as 4,4′-N,N′-dicarbazole-biphenyl (CBP), are likely candidates as triplet energy transfer and exciton confining media. Using 6% to 10%-Ir(ppy)3 in CBP leads to efficient Ir(ppy)3 phosphorescence. In addition to the energetic resonance between the dopant and the host, the control of charge carrier injection and transport in the host layers is believed to be necessary for achieving efficient formation of radiative excitons. High electrophosphorescence efficiency has been achieved using Ir(ppy)3 doped into CBP along with a 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP) electron transport and exciton blocking layer. M. A. Baldo et al., Appl. Phys. Lett., vol. 75, 4 (1999). In that device, the doped CBP layer was found to readily transport holes.
Electrophosphorescent OLEDs are typically comprised of several layers so as to achieve the desired combination of OLED performance characteristics. For example, high efficiencies in organic light emitting devices (OLEDs) may be obtained by differentiating the charge transport and luminescent functions between a host and guest material. A suitable host material may act as a good transporter of charge, as well as efficiently transferring energy to a highly luminescent guest. In fluorescent devices, light may be obtained from singlet excitons formed on the host and rapidly transferred to the guest by Forster energy transfer. Partly owing to this rapid energy transfer, singlet excitons do not diffuse significantly within the host before transferring to the guest material. Thus, OLEDs doped with fluorescent dyes may possess very thin emitting layers, typically approximately 5 nm thick. Tang et al., J. Appl. Phys., vol. 65 (1989) p. 3610.
To obtain electroluminescent emission from more than one emissive material in a fluorescent device, singlet energy transfer may either be retarded, so that some excitons remain on the host material until they relax and emit light; or a multiple-stage energy transfer process, involving several fluorescent dyes, may be employed. Retarded energy transfer is typically an inefficient process and relies on emission from the host. Multiple-stage energy transfer is also possible; however, it may require very precise control over doping concentrations within the about-5-nm-thick luminescent region. Deshpande et al., Appl. Phys. Lett. Vol. 75, No. 7, 888-890 (1999).
In spite of these difficulties in obtaining efficient electroluminescent emission from more than one emissive material, having a plurality of light-emitting dopants within the emissive region of a single organic light emitting device would be very desirable, because the color and intensity of each of the emissive dopants could be tailored to produce a desired output color of light emission, including white light emission. It would be desirable if such devices could be tuned to produce light of a desired color using highly efficient phosphorescent materials.